Regardless if in the disguise of a truck, Megatron would only visually be seen as an Earth vehicle so it seems to be more for the sake of the world than NEST itself. His bio stated that he didn't want to be detected by NEST but doesn't NEST have te technology to determine if a vehicle is a Transformer or not like with Demolisher? Furthermore, don't Transformers emit some sort of radiation as shown by the first movie? So the point I'm getting at is that becoming a truck isn't really necessary because it only make Megatron only less of a target. It would be harder for NEST to find him but it could be easy all the same. He looks cool none-the-less.